Montana Earth Science Picture of the Week

Fossils Revealed As Glacier Melts Away

Photo Courtesy of Callan Bentley, Assistant Professor of GeologyNorthern Virginia Community College

Geologist Callan Bentley lies on the surface of the Grinnell Glacier cirque within several hundred feet of the Continental Divide in this 2007 photo. The strange circular shapes are ancient sea creatures called stromatolites, which until recently, were covered with ice. (photo)

Cabbage Heads . . . The photo to the right was taken just few miles away from the top photo, on the opposite side of the divide along the
Going-to-the-Sun Road in Glacier Park. Although this
location is approximately 5,000 feet above
sea level, it contains fossils of algae colonies.
that lived in a shallow sea .8 to 1.6 billion years
ago. The algae
formed in an environment similar to what
exists in the Florida Keys today. The fossil
forms of these algae, called stromatolites,
have shapes and internal structures similar to
the blue green algae that live in present-day
seas. The outcrop shown in the photo
contains excellent examples of algae colonies
that resemble heads of cabbage (see
close-up below). CLICK HERE to see a photo of modern stromatolites in Shark Bay, Australia.

Gases and Rocks . . . Algae such as these would have taken
carbon dioxide from seawater and released
oxygen as a waste product of photosynthesis.
The algae were a major factor in producing an
oxygen-rich atmosphere, and their removal of
carbon dioxide caused the formation of large
quantities of calcium carbonate. This
contributed to the formation of great
thicknesses of carbonate rocks in the park.

Blame it on Plate Tectonics . . . So how did these sea-dwelling organisms
end up on a mountaintop in Montana? . . .
Blame it on plate tectonics. About 100
million years ago, massive segments of
Earth's crust (crustal plates) moved eastward
from the area of the Pacific Ocean, pushing
into the western edge of North America. This
caused the rocks containing the cabbage
heads to rise from sea level, forming huge
mountains such as those found in this part of
Montana. A similar process is happening
today in the Himalayas where fossils of
ancient sea creatures can be found among
the world's highest mountains.

Source:Geology
Along Going-to-the-Sun Road, Glacier
National Park, Montana, by Omer B.
Raup, Robert L, Earhart, James W. Whipple,
and Paula E. Carrara: prepared by the USGS
in cooperation with the National Park Service,
Published by Glacier National Park Historical
Association in 1983